Human immunodeficiency virus (HIV) is a retrovirus of the Lentivirus family. HIV is a single stranded, positive sense enveloped RNA virus. HIV causes acquired immunodeficiency syndrome (AIDS) which leads to failure of the immune system due to destruction of T cells, macrophages and dendritic cells. HIV has a very high rate of genetic variability. Two types of HIV have been identified, including HIV-1 and HIV-2, both of which are transmitted by sexual contact or through blood and both cause AIDS. The two viruses differ in that HIV-1 is more prevalent, more virulent, and more easily transmitted than HIV-2. HIV-1 can be further divided into three groups based on sequence differences in the envelope (env) gene, group M, group N group O, and group P. Group M of HIV-1 is further divided into at least nine subtypes (or clades) based on difference in genomic sequence and geographic distribution (subtypes A, B, C, D, F, G, H, J and K). The structure of the HIV RNA genome includes nine genes: gag, pol, env, tat, rev, nef, vif, vpr, and vpu. Some HIV genomes include a tenth gene called tev, a fusion of tat, env, and rev. These genes encode the following proteins: